The brake rotor is an integral component of braking systems across multiple types of motor vehicles. The disc brake is an energy conversion device which converts mechanical energy to heat. Disc braking systems consist of a non-rotating friction material and application sub-systems, as well as a brake rotor that rotates with the wheel. To stop or slow the vehicle, the friction material sub-system is engaged with the braking surfaces (rotor cheeks) of the brake rotor to generate heat due to friction, thereby converting mechanical energy to heat, and thereby slowing the rotation of the wheel.
The performance of the braking system, in general, and the brake rotor in particular, is determined to a large extent by the condition of the surface finish of the rotor cheeks. The normal course of operation of a brake system involves production of high levels of friction, which, in turn, generates high temperatures on the rotor cheek surfaces. Environmental effects caused by exposure to corrosive agents, such as road salt and water, exacerbate these problems. These effects, either singly or in combination, can result in pedal pulsation or a corroded braking surface.
The braking system is an aggressive environment for corrosion and high temperature oxidation of cast iron brake rotors. The oxides produced can preferentially spall during normal brake applications. Oxide spalling produces local high spots, which form deep grooves or scoring of the rotor cheek surfaces. These surface features can produce pedal pulsation during braking.
Open style wheel designs, which are currently very popular, leave the rotor braking surfaces visible to onlookers. Surface corrosion that would normally be inconsequential in the operation of the braking system becomes an issue due to the perception of this oxidation.
A variety of methods have been attempted to improve the performance of brake rotor surfaces with respect to oxidation and corrosion. Aluminum rich paint, such as a B90 coating, may be applied to the rotor, but is readily removed during an initial brake application. Ceramic coatings and metallic plates provide corrosion protection, however, these have negative, undesirable braking characteristics.
Gaseous ferritic nitrocarburizing provides a durable corrosion and oxidation resistant diffused case without a large negative effect on braking performance. However, this process may produce geometric distortions that are problematic. Additionally, gaseous ferritic nitrocarburizing may involve long cycle times. Case hardening techniques such as conventional carbonitriding as performed above a critical temperature of the ferrous material, can result in very high distortion, long cycle times and a case structure that is not optimized for corrosion performance.
Therefore, what remains needed in the art is a means of producing a ferritic nitrocarburized surface treatment of cast iron brake rotors, without causing distortions such as thickness variation and lateral run out, which provides corrosion and elevated temperature oxidation resistance.